An urgent and widespread need exists for the remote sensing and recognition of hazardous chemicals in various field environments. However, current techniques available for such purpose are very limited as compared to that of contact or proximal detections. Existing techniques based on laser detecting and ranging (LADAR) and hyper-spectral imaging are cumbersome, sophisticated and expensive and, therefore, are not ideal for the desired quick and widespread deployment. Meanwhile, some highly light-scattering materials such as amorphous solid, powder, gel and suspension are not readily amenable to the conventional optical detection.
It has been reported that an interaction between electromagnetic radiation in the visible spectrum and a material can be detected photoacoustically or photothermally, where the sample temperature increases as a result of light absorption. In the direct photoacoustic effect, when the sample is illuminated with an intensity-modulated chopped light, an acoustic signal is produced at a certain wavelength due to light absorption by the sample molecules. Unfortunately, the direct photoacoustic method is unsuitable for the remote detection of chemicals as it needs a special resonance chamber to amplify the signal, and further requires that the acoustic transducer be physically very close to the sample.
The local temperature of the sample can also be detected using the so-called mirage effect, where a laser beam is passing over the sample when the sample is illuminated by an incident light. The temperature increase leads to a pressure change in the nearby medium, which in turn deflects the laser beam and is detected. However, the detection of deflected laser beam requires a photodetector at variable positions around the sample, which in some field cases is impossible. Although an infrared camera can detect the temperature of the remote sample, its passive nature makes it sensitive to environment variations. In addition sensitivity and spectral resolution of infrared sensors are not sufficient to detect extremely small variations in temperature in the presence of a background created by an illuminating wavelength. In contrast, active sensors generate their energy with known properties and can be used under a wider range of operational condition with fewer constraints.
Today there is a gap between ranges of scales in which imaging techniques can be used. One range is provided by confocal/multiphoton techniques and another range is provided by x-ray/neutron techniques. As materials and structures of nanoscale become of interest, there is a need to identify and provide images of surface molecules and molecules within biological cells.
Various commercial characterization tools such as force modulation microscopy (FMM), nano-indentation, and picosecond ultrasonic and photoacoustic probes, and confocal microscopes address some aspects of samples. Each tool, however, fails to meet one or more of the key criteria regarding spatial resolution, quantitative capability or nondestructive nature. FMM lacks dynamic range for materials with contact stiffness exceeding the cantilever spring constant and is qualitative. FMM lacks dynamic range for materials with contact stiffness exceeding the cantilever spring constant and is qualitative. Nanoindenters are quantitative, but destructive of the samples being analyzed. Far-field ultrasonic microscopy suffers from spatial resolution limitations. Moreover, all far field microscopes lack: resolution limitations due to Rayleigh limit, coupling fluid attenuation ˜f2, and impedance mismatches. Recently, ultrasonic force microscopy has been widely used to map the elastic properties of soft and hard surfaces. It also provides quantitative analysis of surface mechanical properties, but it does not possess sub-surface imaging capabilities and have limited depth resolution of few nanometers.